The present invention relates to a digital wireless communications device, and more particularly, to an improvement technology for increasing call distance, while avoiding mute processing as far as possible, even if a code error occurs in the ADPCM code.
The ADPCM (Adaptive Differential Pulse Code Modulation) method is commonly used as a voice encoding method for digital cordless telephones. One characteristic of the ADPCM encoding method is that, if a coding error occurs in the voice data due to the effects of weak field strength, fading, electromagnetic interference, or the like, then a sudden clicking noise which is disturbing to the ear is generated, and hence the voice quality is markedly degraded. In Japanese Patent Laid-open No. 2002-006890 and Japanese Patent Laid-open No. 2002-006891, the present applicants have proposed a voice quality improvement device for correcting encoding by monitoring internal variables of the ADPCM decoder, predicting the occurrence of click noises, and estimating the bit error position of the ADPCM code.
FIG. 12 is a functional block diagram of a conventional voice quality improvement device. If the occurrence of an error is detected in the reception signal by means of a frame error check, such as a Cyclic Redundancy Check, or the like, then a substitution unit 45 predicts the occurrence of clicking noise if the short term average dms(k) of the absolute value of the ADPCM code l(k) exceeds a prescribed threshold value within a data section previously determined by the judgment time adjusting section 44. In this case, the substitution unit substitutes the ADPCM code l(k) with the code l′(k) which produces a minimum change in the short term average of the absolute value of the ADPCM code l(k). The codes l(k) or l′(k) are input to the ADPCM decoder 46 and are converted into a PCM signal so(k). An F[l] converter 41 converts the code l(k) to the code F[l(k)], in accordance with the correspondence relationships illustrated in FIG. 13, and it outputs same to a short term average calculating section 42. In the short term average calculating section 42, dms(k) is calculated by means of the calculation dms(k)=(1-2−5) dms(k−1)+2−5F[l(k)]. dms(k−1) is obtained by passing the output dms(k) of the short term average calculating section 42 through a delay unit 43. Here, the value of F[l(k)] is defined as shown in FIG. 13.